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Space, time & CosmosLecture 5:

Galaxies, expanding universe and Relativity

Dr. Ken Tsang

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The Sun is currently traveling through the Local Interstellar Cloud in the low-density Local Bubble zone of diffuse high-temperature gas, in the inner rim of the Orion Arm of

the Milky Way Galaxy, between the larger Perseus and Sagittarius arms of the galaxy.

The Orion Arm is a minor spiral arm of the Milky Way galaxy. It is also referred to as the Local Arm, the Local Spur or the Orion Spur.

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Data of the Milky Way Galaxy

Diameter 100,000 light yearsThickness 1,000 light years (stars)Number of stars 200 to 400 billionOldest known star 13.2 billion yearsMass 5.8×1011 M☉

Sun's distance to galactic center 26,000 ± 1,400 light-yearsSun's galactic rotation period 220 million years

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The Milky Way as seen from Death Valley, 2007, a panoramic picture

360° panorama of Racetrack Playa in Death Valley at night. The Milky Way is visible as the arc in the center.

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This image is mosaic of multiple shots on large-format film. It comprises all 360 degrees of the galaxy from our vantage. Photography was done in Ft. Davis, Texas for the Northern hemisphere shots and from Broken Hill, New South Wales, Australia, for the southern portions. Note the dust lanes, which obscure our view of some features beyond them. Infrared imaging reaches into these regions, and radio astronomy can look all the way through with less detail. The very center, however, shows a window to the farther side. In the center, stars are mostly very old and this causes the more yellow color.

360-degree photographic panorama of the galaxy

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This dazzling infrared image from NASA's Spitzer Space Telescope shows hundreds of thousands of stars crowded into the swirling core of our spiral Milky Way galaxy. In visible-light pictures, this region cannot be seen at all because dust lying between Earth and the galactic center blocks our view.

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Wide-angle view of Magellan Clouds and Milky Way (from NOAO image gallery)

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The Milky Way is thought to be a barred spiral galaxy. Messier 109 is one possible analog.

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The Greek philosophers Anaxagoras (ca. 500–428 BC) and Democritus (450–370 B.C.) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars. Aristotle (384-322 B.C.), however, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere." The Arabian astronomer, Alhazen (965-1037 A.D.), refuted this by making the first attempt at observing and measuring the Milky Way's parallax, and he thus "determined that because the Milky Way had no parallax, it was very remote from the earth and did not belong to the atmosphere.“Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it was composed of a huge number of faint stars. In 1750 Thomas Wright, an English astronomer, in his “An original theory or new hypothesis of the universe”, speculated (correctly) that the Galaxy might be a rotating body of a huge number of stars held together by gravitational forces, akin to the solar system but on a much larger scale. The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk.

History of The Milky Way & galaxies

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In a treatise in 1755, Immanuel Kant elaborated on Wright's idea about the structure of the Milky Way. Kant also conjectured that some of the nebulae visible in the night sky might be separate "galaxies" themselves, similar to our own.

The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel (1738 –1822, British astronomer famous for discovering Uranus) in 1785 by carefully counting the number of stars in different regions of the sky. He produced a diagram of the shape of the Galaxy with the Solar System close to the center.

In 1845, Lord Rosse (1800 –1867, British astronomer) constructed a new telescope (the world's largest) and was able to distinguish between elliptical and spiral-shaped nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.

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The sketch made by Lord Rosse of the Whirlpool Galaxy in 1845.

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Rosse named the Crab Nebula, based on an earlier drawing made with his older 36-inch (91cm) telescope in which it resembled a crab.

In the 1840s, Lord Rosse built the Leviathan of Parsonstown, a 72-inch (183-cm), the world’s largest at that time. When the 72-inch telescope was in service, he produced an improved drawing of considerably different appearance, but the original name stuck.

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Kapteyn, a Dutch astronomer, in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by Harlow Shapley, an American astronomer, based on the cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center. Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane, but after Robert Julius Trumpler, an American astronomer, quantified this effect in 1930 by studying open clusters, the present picture of our galaxy, the Milky Way, emerged.

The parsec ("parallax of one arcsecond", symbol pc) is a unit of length, equal to just under 31 trillion kilometres (about 19 trillion miles), or about 3.26 light-years.

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In the 10th century, the Persian astronomer, Abd al-Rahman al-Sufi made the earliest recorded observation of the Andromeda Galaxy, describing it as a "small cloud". Al-Sufi also identified the Large Magellanic Cloud; it was not seen by Europeans until Magellan's voyage in the 16th century. These were the first galaxies other than the Milky Way to be observed from Earth. Al-Sufi published his findings in his Book of Fixed Stars in 964.In 1054, the creation of the Crab Nebula resulting from the SN 1054 supernova was observed by Chinese and Arab/Persian astronomers. The Crab Nebula itself was observed centuries later by John Bevis in 1731, followed by Charles Messier in 1758 and then by the Earl of Rosse in the 1840s.Toward the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest nebulae (celestial objects with a nebulous appearance), later followed by a larger catalog of 5,000 nebulae assembled by William Herschel.

Other galaxies

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In 1917, Heber Curtis had observed the nova S Andromedae within the "Great Andromeda Nebula" (Messier object M31). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our galaxy. As a result he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies.The matter was conclusively settled by Edwin Hubble in the early 1920s using a new telescope. He was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way. In 1936 Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.

Cepheid is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity.Because of this correlation, a Cepheid variable can be used as a standard candle to determine the distance to its host cluster or galaxy. Since the period-luminosity relation can be calibrated with great precision using the nearest Cepheid stars, the distances found with this method are among the most accurate available.

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Photograph of the "Great Andromeda Nebula" from 1899, later identified as the Andromeda Galaxy

A visible light image of the Andromeda Galaxy recently.

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North America Nebula, including Pelican Nebula

The remarkable shape of the emission nebula resembles that of the continent of North America, complete with a prominent Gulf of Mexico.

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Pillars of Creation in the Eagle Nebula (M16): Stars are being born here.

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A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and possibly a hypothetical substance known as dark matter.

A nebula is an interstellar cloud of dust, hydrogen gas and plasma. Originally nebula was a general name for any extended astronomical object, including galaxies beyond the Milky Way. Nebulae often form star-forming regions, such as in the Eagle Nebula. This nebula is depicted in one of NASA's most famous images, the "Pillars of Creation". In these regions the formations of gas, dust and other materials 'clump' together to form larger masses, which attract further matter, and eventually will become big enough to form stars.

Galaxy and nebula

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The Great Debate

The Great Debate, also called the Shapley - Curtis Debate was an influential debate between the astronomers Harlow Shapley and Heber Curtis which concerned the nature of spiral nebulae and the size of the universe. The debate took place on 26 April 1920.

Shapley was arguing in favor of the Milky Way as the entirety of the universe. He believed galaxies such as Andromeda and the Spiral Nebulae were simply part of the Milky Way.

Curtis on the other side contended that Andromeda and other such nebulae were separate galaxies, or "Island universes". He showed that there were more novae in Andromeda than in the Milky Way. From this he could ask why there were more novae in one small section of the galaxy than the others.

whether distant nebulae were relatively small and lay within our own galaxy or whether they were large, independent galaxies.

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Edwin Hubble (November 20, 1889 – September 28, 1953)

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an American astronomer, who profoundly changed astronomers' understanding of the

nature of the universe by demonstrating the existence of other galaxies besides the Milky Way.

He also discovered that the degree of redshift observed in light coming from a galaxy increased in proportion to the distance of that galaxy from the Milky Way. This became known as Hubble's law, and would help establish that the universe is expanding.

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Hubble's arrival at Mount Wilson in 1919 coincided roughly with the completion of the 100-inch Hooker Telescope, then the world's largest telescope. At that time, the prevailing view of the cosmos was that the universe consisted entirely of the Milky Way. Using the Hooker Telescope, Hubble identified Cepheid variables (a kind of star) in several spiral nebulae, including the Andromeda Galaxy. Hubble's observations, made in 1922–1923, proved conclusively that these nebulae were much too distant to be part of the Milky Way and were, in fact, entire galaxies outside our own. This idea had been opposed by many in the astronomy establishment of the time, in particular by Harvard-based Harlow Shapley. His discovery, announced on January 1, 1925, fundamentally changed the view of the universe.

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Some binaries, like Algol, can vary in brightness because of eclipses of one star by the other or gravitational interactions between the two companions.

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A Cepheid variable (or Cepheid) is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity.

Brightness curves for four variable Cepheids over several weeks

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The origin of the name and prototype of these variables is the star Delta Cephei, discovered to be variable by John Goodricke in 1784.

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The relationship between a Cepheid variable's luminosity and variability period is quite precise, securing Cepheids as a viable standard candle and the foundation of the Extragalactic Distance Scale. This period / luminosity connection was discovered in 1912 by Henrietta Swan Leavitt. She measured the brightness of hundreds of Cepheid variables and discovered a distinct period-luminosity relationship.

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Henrietta Swan Leavitt (1868 –1921) was an American astronomer, and the deaf daughter of a Congregational minister. A graduate of Radcliffe College, Leavitt went to work in 1893 at the Harvard College Observatory in a menial capacity as a "computer", assigned to count images on photographic plates. Study of the plates led Leavitt to propound a groundbreaking theory, worked out while she labored as a $10.50-a-week assistant, that was the basis for the pivotal work of astronomer Edwin Hubble and radically changed the theory of modern astronomy, an accomplishment for which Leavitt received almost no credit during her lifetime.

Unaware of her death four years prior, the Swedish mathematician Gösta Mittag-Leffler considered nominating her for the 1926 Nobel prize in physics, and wrote to Shapley requesting more information on her work on Cepheid variables, offering to send her his monograph on Sofia Kovalevskaya. Shapley replied, suggesting that the true credit belonged to his interpretation of her findings. She was never nominated, and the Nobel Prize is not awarded posthumously

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Mount Wilson Observatory, placed in operation in 1904, was the second (after Lick) of the great astronomical research observatories to be established in the Far West. The observatory, at 5,710 feet altitude, is located in the Angeles National Forest on a 1,050-acre plateau at the summit of Mount Wilson.

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In October 1923, Hubble located 3 novae in the Andromeda Nebula, each marked with an ‘N’. One of these turned out to be a Cepheid vriable, so the ‘N’ was crossed out and the star relabeled ‘VAR!’.

Cepheid can be used to measured distance, so Hubble could now measure the distance to the Andromeda Nebula. The result was staggering: Andromeda Nebula is roughly 900,000 light year from the Earth.

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Hubble also developed a classification scheme for galaxies which, with minor revisions remains in use today.

The Hubble sequence: classification of galaxies

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Hubble and Milton L. Humason discovered a rough proportionality of the objects' distances with their redshifts. Though there was considerable scatter (now known to be due to peculiar velocities), Hubble and Humason were able to plot a trend line from the 46 galaxies they studied and obtained a value for the Hubble-Humason constant of 500 km/s/Mpc, which is much higher than the currently accepted value due to errors in their distance calibrations. In 1929 Hubble and Humason formulated the empirical Redshift Distance Law of galaxies, nowadays termed simply Hubble's law, which, if the redshift is interpreted as a measure of recession speed, is consistent with the solutions of Einstein’s equations of general relativity for a homogeneous, isotropic expanding space. Although concepts underlying an expanding universe were well understood earlier, this statement by Hubble and Humason led to wider scale acceptance for this view. The law states that the greater the distance between any two galaxies, the greater their relative speed of separation.

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Historically, spectroscopy referred to the use of visible light dispersed according to its wavelength, e.g. by a prism.

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Emission spectrum of Hydrogen

Emission spectrum of Iron

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Spectrum of a blue sky somewhat close to the horizon pointing east at around 3 or 4 pm on a clear day.

Solar spectrum with Fraunhofer lines as it appears visually.

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Absorption lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to absorption lines in the optical spectrum of

the Sun (left). Arrows indicate redshift. Wavelength increases up towards the red and beyond (frequency decreases).

Redshift (increase in wavelength) and blue-shift (decrease in wavelength)

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Hubble's observations of galaxies with the redshift in their spectral lines.

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Difficulties in Using Cepheids to measure distance

There have been a number of difficulties associated with using Cepheids as distance indicators. Until recently, astronomers used photographic plates to measure the fluxes from stars. The plates were highly non-linear and often produced faulty flux measurements. Since massive stars are short lived, they are always located near their dusty birthplaces. Dust absorbs light, particularly at blue wavelengths where most photographic images were taken, and if not properly corrected for, this dust absorption can lead to erroneous luminosity determinations. Finally, it has been very difficult to detect Cepheids in distant galaxies from the ground: Earth's fluctuating atmosphere makes it impossible to separate these stars from the diffuse light of their host galaxies.

Another historic difficulty with using Cepheids as distance indicators has been the problem of determining the distance to a sample of nearby Cepheids. In recent years, astronomers have developed several very reliable and independent methods of determining the distances to the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), two of the nearby satellite galaxies of our own Milky Way Galaxy. Since the LMC and SMC contain large number of Cepheids, they can be used to calibrate the distance scale.

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He inferred that galaxies were similar to each other in size so those that appeared smaller must be further away. By plotting the velocity of the galaxies against their distance he came across an interesting relationship. This is now known as Hubble's law and is shown in the following plot.

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If you study the above plot you will see that the more distant a galaxy is, on average, the faster it is receding from us. In fact Hubble realised he could fit a linear relationship to his data, as shown by the pale blue line of best fit. The slope of this line is a constant and is now known

as the Hubble constant, H0. This relationship is expressed mathematically as:

v ∝ dwhere v is the recession velocity and d is the distance.

Hubble's law

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The law was first formulated by Edwin Hubble in 1929 after nearly a decade of observations. It is considered the first observational basis for an expanding universe and today serves as one of the pieces of evidence most often cited in support of the Big Bang.

Hubble's law is the statement that the redshift in light coming from distant galaxies is proportional to their distance.

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How the Hubble’s law can be explained by an expanding Universe?

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What is the Big Bang?The "Big Bang" is the term given to what is currently the most widely accepted scientific model for the origin and evolution of the Universe.

This model has supplanted other models such as the Steady State theory proposed by Hoyle, Bondi and Gold in the 1940s. Indeed it was Fred Hoyle who coined the term "big bang" as a derisory one in an interview in the 1960s.

In the Big Bang theory the Universe comes into existence, creating time and space. Initially the Universe would have been extremely hot and dense. It expanded from a primordial hot and dense initial condition and cooled gradually. Some of the energy involved was turned into matter. Current observations suggest an age for the Universe of about 13.7 billion years.

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According to the Big Bang model, the universe expanded from an extremely dense and hot state and continues to expand today.

A common and useful analogy explains that space itself is expanding, carrying galaxies with it, like raisins in a rising loaf of bread.

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The Big Bang Model is supported by a number of important observations, each of which are described in more detail on separate pages:

The expansion of the universe Edwin Hubble's 1929 observation that galaxies were generally receding from us provided the first clue that the Big Bang theory might be right.

The abundance of the light elements H, He, Li The Big Bang theory predicts that these light elements should have been fused from protons and neutrons in the first few minutes after the Big Bang.

The cosmic microwave background (CMB) radiation The early universe should have been very hot. The cosmic microwave background radiation is the remnant heat leftover from the Big Bang.

These three measurable signatures strongly support the notion that the universe evolved from a dense, nearly featureless hot gas, just as the Big Bang model predicts.

Tests of Big Bang Cosmology

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The Big Bang model was a natural outcome of Einstein's General Relativity as applied to a homogeneous universe.

However, in 1917, the idea that the universe was expanding was thought to be absurd. So Einstein invented the cosmological constant as a term in his General Relativity theory that allowed for a static universe.

In 1929, Edwin Hubble announced that his observations of galaxies outside our own Milky Way showed that they were systematically moving away from us with a speed that was proportional to their distance from us. The more distant the galaxy, the faster it was receding from us. The universe was expanding after all, just as General Relativity originally predicted! Hubble observed that the light from a given galaxy was shifted further toward the red end of the light spectrum the further that galaxy was from our galaxy.

The expansion of the universe

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The expanding raisin bread model below illustrates why the specific form of Hubble's expansion law (speed of recession is proportional to distance ) is important. If every portion of the bread expands by the same amount in a given interval of time, then the raisins would recede from each other with exactly a Hubble type expansion law. In a given time interval, a nearby raisin would move relatively little, but a distant raisin would move relatively farther - and the same behavior would be seen from any raisin in the loaf.

In other words, the Hubble law is just what one would expect for a homogeneous expanding universe, as predicted by the Big Bang theory. Moreover no raisin, or galaxy, occupies a special place in this universe - unless you get too close to the edge of the loaf where the analogy breaks down.

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The term nucleosynthesis refers to the formation of heavier elements, atomic nuclei with many protons and neutrons, from the fusion of lighter elements.

The Big Bang theory predicts that the early universe was a very hot place. One second after the Big Bang, the temperature of the universe was roughly 10 billion degrees and was filled with a sea of neutrons, protons, electrons, anti-electrons (positrons), photons and neutrinos. As the universe cooled, the neutrons either decayed into protons and electrons or combined with protons to make deuterium (an isotope of hydrogen). During the first three minutes of the universe, most of the deuterium combined to make helium.

Trace amounts of lithium were also produced at this time. This process of light element formation in the early universe is called “Big Bang nucleosynthesis” (BBN).

Tests of Big Bang: The Light Elements

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George Gamow, a Ukrainian-born, US-based physicist and former student of Friedmann's, made his mark early by applying quantum theory to explain how alpha particles can be ejected from nuclei in alpha decay. Moving from the USSR in 1931 he settled in the US and continued his work on stellar evolution and beta decay. He was particularly interested in trying to solve the problem about the origin of the elements.

Hans Bethe had already shown in the 1930s how helium could be synthesised inside stars through fusion of hydrogen nuclei. He had also explained how protons and neutrons added to carbon nuclei could form heavier elements.Gamow had realised from Hubble's work that the early Universe must have been much smaller, hotter and denser than it is now. In the late 1940s with his students Ralph Alpher and Robert Herman he calculated that helium could form from the fusion of protons (that is, hydrogen nuclei) and neutrons. This nucleosynthesis would cease once the available neutrons were used up and the Universe had expanded and cooled sufficiently.

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They also realised that the Universe should be filled with background microwave radiation, the remnant of the original big bang now cooled to about 50 Kelvin. This

radiation would have the spectral characteristics of a blackbody.

Gamow's theory of the nucleosynthesis of primordial helium accounted for the observed abundance of helium compared with hydrogen in the Universe whereas stellar nucleosynthesis could not.

His prediction of remnant radiation was neglected by others until the 1960s but was to provide the key evidence in support of the big bang model for the Universe.

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Evidence for the Big Bang Model:Cosmic Microwave Background Radiation

In 1965 two scientists working for Bell Telephone Laboratories, Arno Penzias and Robert Wilson were adapting a horn-shaped antenna near New York for use in radio astronomy. They encountered noise in the system and despite repeated and thorough attempts were unable to remove it or find its cause. They eventually

realized that this "noise" was in fact remnant radiation from the big bang.

Such radiation had been predicted by Gamow in the late 1940s. As the Universe expanded it cooled so that today the background radiation corresponds to a temperature of 2.725 K and has a black body spectrum.

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This plot shows the black-body nature of the cosmic microwave background radiation. The spectrum corresponds to background radiation with a temperature of 2.725 K.

These measurements were made by the FIRAS instrument on the COBE satellite. The error bars for each measurement are smaller than the width of the red line.

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This diagram shows how the peak wavelength and total radiated amount vary with temperature. Although this plot shows relatively high temperatures, the same relationships hold true for any temperature down to absolute zero. Visible light is between 380 to 750 nm.

Black-body/Thermal radiation is electromagnetic radiation emitted from the surface of an object due to the object's temperature. Infrared radiation from a common household radiator or electric heater is an example of thermal radiation, as is the light emitted by a glowing incandescent light bulb.

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Example of Black-body/Thermal radiation

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Welcome to the Universe: Nebula & Galaxies: A Cosmic Journeyhttp://www.youtube.com/watch?v=X5zVlEywGZg&NR=1Stephen Hawking's Universe - EP1:Seeing Is Believing (1/ 5) http://www.youtube.com/watch?v=jd1tgLQg4ZU&feature=related Stephen Hawking's Universe - EP1:Seeing Is Believing (2/ 5) http://www.youtube.com/watch?v=PJamA_ulJ50&feature=related Stephen Hawking's Universe - EP1:Seeing Is Believing (3/ 5) http://www.youtube.com/watch?v=HFe1BL3gvo0&feature=related Stephen Hawking's Universe - EP1:Seeing Is Believing (4/ 5) http://www.youtube.com/watch?v=MxTCFkP-snI&feature=related Stephen Hawking's Universe - EP1:Seeing Is Believing (5/ 5) http://www.youtube.com/watch?v=sSfrbYyQIww&feature=related

Vidoe to watch (and report):

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Stephen Hawking's Universe - EP2: The Big Bang (1/ 5) http://www.youtube.com/watch?v=MZa7px6NtFY&feature=related Stephen Hawking's Universe - EP2: The Big Bang (2/ 5) http://www.youtube.com/watch?v=Rc2hNHjC84Q&feature=related Stephen Hawking's Universe - EP2: The Big Bang (3/ 5) http://www.youtube.com/watch?v=iv48uVZ2vnk&feature=related Stephen Hawking's Universe - EP2: The Big Bang (4/ 5) http://www.youtube.com/watch?v=IFPzBMTOnxM&feature=related Stephen Hawking's Universe - EP2: The Big Bang (5/ 5) http://www.youtube.com/watch?v=sUyrnvmg6zU&feature=related